Electron ribbon beam encoder tube with beam tilt control



Sept. 29, 1964 M. H. CROWELL ELECTRON RIBBON BEAM ENCODER TUBE WITH BEAMTILT CONTROL 2 Sheets-Sheet 1 Filed March 31, 1961 lNl/ENTOR M H.CROWELL V TTOPNE Sept. 29, 1964 M. H, CROWELL ELECTRON RIBBON BEAMENCODER TUBE WITH BEAM TILT CONTROL 2 Sheets-Sheet 2 Filed March 31,1961 FIG. 4

INVENT M, H. mow/ELL B! A 7 TOR United States Patent "ice 3,151,270ELECTRON RIBBQN BEAR ll ENCQBER TUBE WITH BEAM TILT CQNTROL Merton H.Crowelii, Morristown, Nail, assignor to Eeil Telephone Laboratories,incorporated, New York, N.Y., a corporation of New York Filed Mar. 31,1961, Ser. No. 99,945 9 Claims. (Cl. 315-2l) This invention relates toelectron beam tubes and more particularly to electron beam tubes inwhich the beam must be maintained in alignment with other elements inthe tube while being deflected with respect thereto.

Electron beam tubes are of various types which may be employed in manydifferent types of systems and circuits. For example, such tubes may beused as beam deflection amplifiers, cathode ray devices, or encoders.Although in certain aspects the present invention is not limited inapplication to any one type of electron beam tube, it will be describedherein with particular reference to electron beam encoding devices,which generally comprise an electron gun for producing an electron beam,deflection plates for changing the direction of the beam in response toan input signal, a plurality of targets upon which the beam may impinge,and an apertured code plate positioned adjacent the targets between thetargets and the deflection plates.

A highly successful type of electron beam encoder is the ribbon beam orflash coder described in R. L. Carbrey Patent 2,516,752. The flash coderemploys a flat or ribbon-like electron beam with a line focus whichimpinges on an apertured code plate. The apertures on the code plate arearranged in rows and columns, each column corresponding to a digit in abinary code and each row corresponding to different combination of thebinary digits. Associated with each column is a collector electrodewhich is energized whenever a part of the electron beam passes throughan appropriate aperture. The code plate is aligned in the encoder tubeso that the aperture columns are perpendicular, and the aperture rowsare parallel, to the plane of the ribbon beam. Thus, as the beam isdeflected in accordance with varying signal amplitudes the set ofcollector electrodes is energized in different combinationscorresponding to the various levels in the code.

The principal applications of electron beam encoding tubes is in pulsecode modulation systems. Such tubes have been built which are capable ofencoding samples per second into a seven-digit binary code. However,studies of presently available transmission systems indicate a need forencoders capable of converting analog signals into digital informationat sampling rates higher than 10 megacycles per second and with anaccuracy or resolution substantially better than one part in twohundred. Such a degree of resolution requires a code of at least eightand preferably nine binary digits.

While the electron beam encoder as heretofore known is, in principle,well adapted for high speed, high resolution applications, a number ofpractical limitations have so far prevented such use. In particular, thedevelopment of a satisfactory beam-type encoder for encoding eight ornine binary digits has been inhibited by the fact that an increase inthe number of digits in these devices is accompanied by a drasticnarrowing of the mechanical and electrical tolerances which must be metto give the required accuracy. In general, the effect of adding onedigit to a given encoding system is to halve the critical tolerances inthe tube. Inasmuch as the tolerances in a seven-digit encoding tube ofthe best type hitherto known approach the limits of practicality, thefeasibility of producing tubes capable of higher resolution is dependenttilt.

Patented Sept. 29, 196

on the discovery of new and different techniques for achieving orrelieving these stringent requirements.

Among the critically important mechanical requirements of a beam typeencoder capable of encoding a greater number of digits than previousencoder tubes is a code plate with precisely dimensioned and preciselyspaced apertures. In a typical encoder tube of the type underconsideration, the effective area of the code plate is 2.05 by 0.5inches. Within this limited area must be arranged seven, eight, nine ormore columns of apertures. In a nine-digit tube, there may be 128apertures in the finest digit column. It is apparent that the size andspacing of the apertures would be less critical if a larger code platewere used. A number of factors, however, have served to limit the sizeof the code plates heretofore used. For instance, in a particular tubein which the distance from the deflection system to the code plate isfixed, the use of a larger code plate must be accompanied by acorresponding increase in the deflection sensitivity of the tube, ormore powerful deflection driving circuits must be provided.

Priorly known electron beam-type encoders, however, have beencharacterized by a fairly low deflection sensi tivity. Thecharacteristics of such tubes have in fact been such as to necessitatethe use of vacuum tube deflection amplifiers in otherwise completelytransistorized systems. For instance, in a pulse code video transmissionsystem described by R. L. Carbrey at page 1546 of Proceedings of theI.R.E. for September, 1960, a vacuum tube ampliher was required to drivethe encoder tube although the rest of the system was entirelytransistorized. The considerable advantages derivable from the use oftransistor circuitry throughout such systems will be apparent to workersin the art.

A larger'code plate may be employed in an encoder tube without adverselyaffecting the deflection sensitivity if the distance between thedeflection systems and the code plate is increased. This expedient is,however, subject to a number of objections. In particular, it wouldenlarge the size of the tube envelope, thus making it more difiicult tomanufacture as Well as moreliable to structural failure. Furthermore,the larger the volume inside the envelope, the more difficult it is tocontrol the environment of the electron emissive cathode surface. Largertubes typically have a higher rate of cathode failure due tocontamination of the emitter surfaces. In addition, such a tube wouldrequire a longer electron beam. It is well known however, that, due tospace charge effects, the maximum current which may be focussed in abeam of given size varies inversely as the square of the distance to thefocus. Increasing the length of the tube, therefore, is to be avoidedwhenever possible.

A further source of difficulty in prior encoding tubes arises from theneed for exact alignment of the electron beam and the code plate. Forinstance, in one advantageous arrangement the rows of apertures in thecode plate are perpendicular to the columns thereof. In order toeliminate a major source of encoding error in a tube comprising such acode plate, it is essential that the ribbon beam be parallel to the rowsof apertures. A beam not so aligned is said to be tilted.

It has been recognized in the prior art that a major source of beam tiltin electron beam-type encoders is misalignment of the electron gun andthe code plate. As these two components are at opposite ends of a glassenvelope which typically is rather large, uniform and accuratemechanical alignment is difficult to achieve. The error resulting frommisalignment is independent of the angle of deflection and may bereferred to as .static Accordingly, it has been found advantageous toprovide electrostatic means for aligning the electron beam with the codeplate, thereby avoiding the necessity for precise mechanical alignment.This may be accomplished by including in the encoder tube a pair of tiltcorrection plates as described in United States Patent 2,713,650 to- R.W. Sears.

A hitherto uncompensated source of beam tilt is misalignment of theelectrostatic deflection plates in the main beam-deflection system ofthe encoder tube. In an idealized deflection system in which the platesare absolutely planar and perfectly parallel, the tangent of the angleof deflection is directly proportional to the voltage across the platesand inversely proportional to their separation. It can be shown that theangle of beam tilt caused by a small change in separation of the platesacross the beam width is dependent on the angle of deflection of thebeam. That is to say, unlike the statictilt caused by misalignment ofthe electron gun and the code plate, the tilt caused by lack of perfectparallelism of the deflection plates is a function of the angle ofdeflection. This source of encoding error may be designated dynamictilt.

The effect of encoding errors such as those caused by beam tilt is toincrease the noise in the coded signal over that which would be presentin a signal coded by a perfect encoder system using the same number ofdigits. The degradation of the signal due to coding errors may also beexpressed as a reduction in the eflective number of digits in thesystem. The adverse effect of dynamic beam tilt becomes progressivelymore serious as the number of digits in the code is increased. Forexample, in. order to maintain the error from this source at anacceptable level in an eight-digit tube of typical design, the maximumallowable error in deflection plate separation is about 0.0050 inch. Fora nine-digit tube of the same type the maximum allowable error is halfthat of the eight-digit tube or about 0.0025 inch. While it may bepossible to build a deflection system that would meet theserequirements, such precision renders the cost prohibitive and the designimpractical.

An object of this invention, therefore, is to increase the deflectionsensitivity of electron beam tubes without materially increasing thedistance between the deflection system and the target.

A second object of this invention is to maintain proper alignment of anelectron beam with another element in an electron beam tube as the beamis angularly deflected from a position in which it is properly aligned.

A third object of this invention is to increase the encoding accuracy offlash coding devices by maintaining proper alignment between theelectron beam and the apertured code plate as the beam is deflected froma position in which it is properly aligned.

A further object of the invention is to improve the performance of flashcoders employing ribbon electron beams, and to make feasible theconstruction of flash coders capable of encoding a greater number ofquantized levels than prior beam-type encoders.

Yet another object of this invention is the accurate encoding of analogsignals in multi-digit binary codes by means of an electron beam-typeencoder capable of being driven by transistorized circuits.

These and other objects of the invention are achieved in a specificillustrative embodiment comprising, in an evacuated envelope, anelectron gun for projecting a ribbon electron beam against a target,electrostatic deflection plates positioned between the gun and thetarget for deflecting the electron beam in a direction perpendicular tothe plane of the ribbon beam, dynamic tilt correction means positionedbetween the electron gun and the deflection plates, and an electrostaticdiverging lens positioned between the deflection plates and the targetfor increasing the deflection sensitivity of the beam deflection system.

A feature of this invention is an electrostatic diverging lenscomprising two axially-spaced plane parallel conductive members havingsimilar beam-passing apertures,

and a grid of very small closely spaced parallel wires defining aplurality of elongated apertures. The grid is positioned between theparallel conductive members with the elongated apertures aligned in thedirection in which the divergence is to be produced. The two parallelmembers are maintained at one potential, and the grid is maintained at alower potential. Beam divergence occurs in one dimension only, that is,in the dimension defined by the elongated apertures of the grid. Thus ifthe beam is deflected so that it is obliquely incident on the diverginglens, the lens acts to increase the angle of deflection, therebyincreasing the elfective deflection sensitivity of the tube.

It is another feature of this invention that a dynamic tilt correctionsystem in an electron beam tube employing a ribbon beam comprises a pairof conductive members positioned on opposite sides of the ribbon beamand extending across the width thereof. Advantageously, the conductivemembers are uniformly tapered rods of circular cross section. In onepreferred embodiment the axes of the tapered rods are parallel acrossthe width of the ribbon beam. Alternatively, the conductive members maybe rods of circular cross section and constant diameter, in which casethey are arranged with a uniform variation in separation across thewidth of the beam.

It is a further feature or" this invention that the conductive membersin the dynamic tilt correction system are connected to the main beamdeflecting plates through an attenuating network, whereby there isestablished between said members a potential difference which is afunction of the primary beam deflecting voltage. The attenuating networkis preferably of a frequency compensated type so that the amount ofdynamic tilt correction applied to the beam is substantially independentof the frequency with which the beam is deflected. Thus as a voltage isapplied to the deflection plates, thereby deflecting the beam andcausing it to be undesirably tilted or rotated, a second derived voltageis applied to the correction system. The correction voltage pretilts theplane of the beam in a direction opposite the tilt produced by themisaligned deflection plates so that the two rotations cancel and thebeam passes untilted to the code plate.

The above-mentioned and other objects and features will be fullyunderstood from the following more detailed discussion taken inconjunction with the accompanying drawing in which:

FIG. 1 is a view of a ribbon beam encoding tube illustrative of theinvention, with a portion of the envelope cut away to reveal theinterior structure;

FIG. 1A is a side view of a portion of the interior structure of thetube shown in FIG. 1;

FIG. 2 is a partial sectional view of the tube structure taken along theline 2-2 of FIG. 1A;

FIG. 3 is a partial sectional view of the tube structure taken along theline 33 of FIG. 1A;

FIG. 4 is a diagrammatic representation of an electron beam tube havingan electron beam diverging lens according to the invention; and

FIG. 5 is a schematic representation of an electron beam tubeillustrative of the invention, showing one combination of circuitelements employable therewith.

Similar elements are indicated by like reference numerals throughout thedrawing.

Turning now to FIG. 1, the specific illustrative embodiment there showncomprises an envelope 12 which may be of a convenient material such asglass. The envelope is evacuated during manufacture in any suitablemanner known in the art. Advantageously, the exhaust tribulation, notshown, is enclosed in a base 11 having lead terminals 16 extendingtherethrough for making electrical contact to various elements containedwithin the envelope.

An electron gun assembly, not shown, of a type suitable for forming aflat or ribbon-shaped electron beam is 55 positioned in the base end ofthe envelope. An anode 13 is mounted on insulated support rods 14- whichextend along the axial dimensions of the tube. Also mounted on the rods14 are beam-forming and accelerating electrodes 16, which may be in theform of apertured discs.

In the specific illustrative embodiment shown, a dynamic tilt correctionsystem in accordance with the invention comprises a pair of taperedconductive members 20 secured by metal brackets 1'7 which are supportedon the rods 14. The form of the brackets 17 and the mounting of thetapered members 20 may be more clearly seen by reference to the sideview in FIG. 1A and to the sectional view in FIG. 3. The tapered members20 are electrically insulated from the brackets 17 and from each otherby insulating bushings 33 which may be, for example, of a ceramicmaterial. A portion of each member 20 protrudes beyond the bushing 33 inone of the brackets 17 to afford means for making electrical connectionto each of said members individually. The individual conncctions arecarried through the envelope 12 by lead terminals, such as lead terminal18. In the interest of clarity the wires connecting the members 20 tothe terminals 18 have been omitted from the drawing. A second leadterminal would extend through the portion of the envelope 12 which inFIG. 1 is cut away to reveal the interior structure of the tube.

Although the conductive members 20 in the illustrative embodiments arein the form of tapered or frusto-conical rods, other forms are possibleand may be employed within the spirit of the invention. Thus the members20 may comprise cylindrical rods having a varying separation across thewidth of the electron beam. More complex shapes may be devised by thoseskilled in the art to approximate the ideal configuration in which thefield varies linearly across the Width of the beam. Such shapes are alsowithin the scope of the invention. However, the use of tapered rods orrods having a uniformly varying separation across the beam width ispreferred because of the ease and low cost of fabrication and assembly,and because they add least to the length of the tube structure. Asexplained above, it is most desirable to make the tube as short aspossible.

In one specific electron beam tube comprising a dynamic tilt Correctoras described above, the length of the tapered members 20 betweenbrackets 17 was 0.750 inch. The members 20 were of circularcross-section tapered from a diameter of 0.100 inch at one end to adiameter of 0.025 inch at the other end. The distance between centerswas 0.200 inch.

In a second specific tube the dynamic tilt corrector comprised twocylindrical rods of diameter 0.100 inch and length 1.00 inch. Thedistance between centers was 0.200 inch at one end and 0.250 inch at theother end. It may be shown that such an arrangement will rotate theplane of the ribbon beam by an amount where V is the potentialdifference between the rods, V is the energy of the electrons in thebeam, and Z is the distance between the dynamic tilt corrector and thetarget assembly. If Z is about 6 inches and V is about 1,000 volts, thetilt sensitivity is about 0.1 degree per volt. Of course, thesensitivity may be varied by changing the parameters of the dynamic tiltcorrector, but the above has been found suflicient to ease themechanical requirements imposed on the deflection system to a pointwhere manufacture is relatively easy and inexpensive. Moreover, the useof the beam diverging lens which is a part of this invention results ingreater effective sensitivity of the corrector. In the preferredarrangements the parameters are adjusted so that the maximum voltagerequired by the dynamic tilt corrector is no greater than the maximumvoitage applied to the main deflection systern, so that the combinedbeam controlling means may be driven by a single source.

Also supported by the rods 14 are deflection plates 21 which, as can beseen from FIG. 1A, are advantageously closer together at the base end ofthe tube than at the target end. Such an arrangement, as is known in theart, results in greater deflection sensitivity. In addition it has beenfound advantageous to place a shield electrode 19 between the dynamictilt corrector and the deflection plates 21 to reduce the interactionbetween the time varying electric fields produced in the adjoiningportions of the electron beam path. The shield electrode 19 may be inthe form of an apertured conductive disc.

Adjacent the deflection plates 21 on the side nearest the target end ofthe tube is situated an electron beam diverging lens in accordance withthe invention. As shown in FlGpl the diverging lens comprises a planargrid 22 and a planar electrode 23 in spaced parallel relation therewith.The grid 22 comprises a set of very small closely spaced conductivemembers 32 and is illus trated in greater detail in FIG. 2. The members32. form a plurality of elongated apertures which, advantageously, arealigned in the direction in which the beamdivergence is to be produced.In the tube shown in FIG. 1 the ribbon-shaped electron beam is to bedeflected in a direction normal to the plane thereof, so that the grid23 is aligned in a plane normal to that of the beam.

The planar electrode 2.3 may be in the form of a disc having arectangular beam passing aperture 25 therethrough. In order to minimizethe effect of electric field distortion near the ends of the aperture,it is desirable that the length thereof be as great as is convenient.Advantageously, the aperture 25 extends across the major portion of thedisc 23, suflicient metal being left at the edges of the disc tomaintain its shape. Furthermore, it may be desirable in some instancesto extend the aperture 25 across the entire width of the disc 23, sothat the latter is divided into two separate and independently supportedsections which may be electrically connected by a wire at the edge ofthe gap between them.

It is preferred that the elongated apertures of the grid 22 beterminated at as great a distance as is possible from that region of thegrid through which the electron beam actually passes. Such aconfiguration minimizes the aberrational and defocussing eifects of thegrid apertures in the direction normal to the plane of the ribbon beam.This objective is conveniently obtained by shaping the grid 22 toconform in general to the crosssectional shape of the tube envelope.Thus, in the cylindrical tube shown in FIG, 1, the grid 22 is circularand the apertures therein extend almost to its periphery.

v In a specific electron beam tubeembodying this aspect of theinvention, the grid 22 was 2 inches in diameter. The aperturestherethrough were about 0.0017 inch wide and were defined by parallelconductive members 32 which were about 0.0003 inch thick. The grid wasplaced 0.250 inch from the end of the deflection plates 21 and 0.500inch from the lens electrode 23.'-This tube was operated with apotential difference of 300 volts between the grid 22 and the electrode23. The gain in deflection sensitivity amounted to about 50 percent.

The apertured grid 22 was formed by winding .0003 inch diameter tugnstenwire about an annular member, brazing the wire thereto, and cutting theannular member to divide it into two similar planar grids. Othertechniques for forming the lens grid will appear to those skilled in theart.

Although the beam diverging lens as shown in FIGS. 1 and 2 comprises aplanar grid and a single planar electrode in spaced parallel relationtherewith, several variations are possible within the scope of theinvention. Thus the electrode 23 as shown in FIG. 1 is situated betweenthe grid 22 and the collector shield 24. Inasmuch as the lens isoperated with the electrode at a positive potential with respect to thegrid, this will be designated the accelerating configuration. Theelectrode 23 may, however, be positioned between the grid 22 and thedeflection plates 21. This will be referred to as the deceleratingconfiguration. In a third variation, shown schematically in FIG. 5, thegrid 22 is located between two planar electrodes 23. This may bedesignated the unipotential configuration, although different potentialsmay be applied to the two electrodes.

PEG. 4 illustrates schematically the manner in which a beam diverginglens acts to increase the deflection sensivity of an electron beam tube.An electron gun 41 produces a beam 42 which is deflected by an angle 11/as it passes through the electric field between the deflection plates21. The path of the beam 42 is bent again as it passes through the lenscomprising grid 22 and electrodes 23, the diverging effect beingindicated by the angle A 0. The gain in deflection sensitivity may bereprewhere Y is the amount of deflection at the target 29 with the lens,Y is the amount of deflection without the lens, 1 is the focal length, Zis the distance from the lens to the target 29, and Z is the disatncefrom the center of deflection to the target.

The manner of operating an electron beam tube embodying the inventionmay be understood by referring to FIG. 5. A stream of electrons emittedby the cathode 51 under the influence of anode l3 and the controlelectrode 52 is formed into a flat or ribbon-like beam 42 by theaccelerating and beam-forming electrodes 16. A voltage source 53 acts incombination with resistors 54, 55 and S6, and with capacitors 56 and 57to maintain appropriate potential diflerences between the variouselements of the electron gun. It is to be understood that any suitabletype of electron gun may be used to form the electron beam, and that thegun shown in FIG. is by way of illustration only.

In the preferred embodiment of the invention the dynamic tilt correctorcomprising the members 20 is characterized by a tilt sensitivity suchthat the voltage required at any angle of deflection is less than thevoltage applied to the deflection plates 21. The tilt correction voltageis then conveniently derived directly from the deflection plates throughan attenuating network 59. Advantageously, the attenuator 59 isfrequency compensated so that the proper potential is supplied to themembers 20 irrespective of the frequency with which the beam isdeflected. Networks of this type are known in the electronics art.

In the event that the tilt sensitivity of the rods 2b is insuflicient topermit the use of an attenuating nework as described, it will benecessary to provide a voltage amplifier or other means for supplyingthereto a voltage dependent on the deflection voltage. In either casethe rods 28 are to be connected to the potential supplying means so thatthey produce a dynamic tilt or beam rotation opposite in direction tothat caused by misalignment of the deflection plates 21. The properpolarity of the connection will vary from tube to tube due to therandomness of the errors introduced in the manufacturing process, andmay be determined empirically after the ribbon beam 42 has beenstatically aligned with the target assembly 29.

After being deflected by the field between the plates 21 the beam 42passes through the diverging lens which, in FIG. 5 is of theunipotential type. In this type of lens the electrodes 23 preferably areboth maintained at the same positive potential with respect to the grid22 by means of the bias source 64 in combination with resistors 66 and67. The diverging effect of the lens is modified if the electrodes 23are maintained at different positive potentials respectively.

It can be seen from the above specification that the inclusion, in anelectron beam encoder tube, of a dynamic tilt correction system and abeam diverging lens permits the construction of such a tube capable ofencoding a greater number of signal levels and having an improveddeflection sensitivity, all without the drastic narrowing of mechanicaland electrical tolerances which would otherwise be required.

It will also be appreciated by workers in the electron beam art that thedynamic tilt corrector may be used with pencil beam which must bedeflected and swept across a target in alignment therewith. Furthermore,the diverging lens in accordance with the invention may be employed inother beam devices where one dimensional divergence is desirable.

What is claimed is:

1. An electron beam tube comprising target means, an electron gun forprojecting a ribbon electron beam in an initial plane toward said targetmeans, electrostatic deflection means intermediate said gun and saidtarget means for deflecting said beam away from said plane in adirection perpendicular to said plane, electrostatic diverging meansintermediate said deflection means and said target means for causingsaid beam to diverge in the direction of deflection, dynamic tiltcorrection means intermediate said gun and said deflection means formaintaining the alignment of said beam and said target as said beam isdeflected, said tilt correction means comprising conductive memberspositioned on opposite sides of the plane of said ribbon electron beam,and means for supplying to said dynamic tilt correction means potentialsof a magnitude functionally dependent on the amount by which said beamis deflected.

2. An electron beam tube as in claim 1 wherein said electrostaticdiverging means comprises first and second axially-spaced plane parallelconductive members having first and second similar beam-passingapertures respectively, said apertures being so dimensioned as to passthe entire thickness of the ribbon beam at all deflections of said beam,a planar grid between said members in plane parallel spaced relationthereto, said grid comprising a set of very small closely-spacedparallel conductors defining a plurality of elongated apertures, thelong dimension of said apertures being perpendicular to said plane, andmeans for maintaining said first and second axially spaced members at apositive potential with respect to said grid.

3. An electron beam tube as in claim 1 wherein said electrostaticdiverging means comprises at least one planar conductive member having abeam-passing aperture so dimensioned as to pass the entire thickness ofthe ribbon beam at all deflections of said beam, a planar grid spacedfrom said member in plane parallel relation thereto, said gridcomprising a set or" closely-spaced parallel conductors defining aplurality of elongated apertures, the width of said conductors beingsmaller than the separation between them, the long dimension of saidapertures being perpendicular to said plane, and means for maintainingsaid planar member at a positive poten tial with respect to said grid.

4. An electron beam tube as in claim 1 wherein said dynamic tiltcorrection means comprises a pair of conductive members positioned onopposite sides of the plane of said electron beam and having a uniformlyvarying separation across the width thereof, and means for applying tosaid members a potential ditference proportional to the angle by whichsaid beam is deflected from a reference position.

5. An electron beam tube as in claim 4 wherein said conductive membersare cylindrical rods.

6. An electron beam tube as in claim 1 wherein said dynamic tiltcorrection means comprises a pair of uniformly tapered conductive rodsof circular cross-section, said rods being positioned on opposite sidesof the plane of said electron beam with their axes parallel andextending across the width thereof, and means for applying to said rodsa potential diflerence proportional to the angle by which said beam isdeflected from a reference position.

7. An electron beam tube comprising target means, an electron gun forprojecting a ribbon electron beam in an initial plane against saidtarget means, deflection means intermediate said gun and said targetmeans for angularly deflecting said beam away from said plane in adirection perpendicular to said plane, dynamic tilt means intermediatesaid deflection means and said target means for tilting said beam by anamount dependent on the angle by which said beam is deflected, dynamictilt means comprising a pair of tapered rods positioned on oppositesides of the plane of said beam with their axes parallel in a planeperpendicular to the direction of said beam, and means for applying tosaid rods a potential difference proportional to the angle by which saidbeam is deflected from a reference position.

8. An electron beam tube comprising target means, an electron gun forprojecting a ribbon electron beam in an initial plane against saidtarget means, deflection means intermediate said gun and said targetmeans for angularly deflecting said beam away from said plane in adirection perpendicular to said plane, dynamic tilt means intermediatesaid deflection means and said target means for tilting said beam by anamount dependent on the angle by which said beam is deflected, saiddynamic tilt means comprising a pair of conductive rods of uniformcrosssection, said rods being positioned on opposite sides of the planeof said beam and having a uniformly varying separation in a planeperpendicular to the direction of said beam, and means for applying tosaid rods a potential difference proportional to the angle by which saidbeam is deflected from a reference position.

9. An electron beam encoder tube comprising a plurality of targetelectrodes, an electron gun for projecting a ribbon electron beam towardsaid target electrodes, a code plate situated between said electron gunand said target electrodes, said code plate having a plurality of beampassing apertures therethrough for permitting combinations of saidtarget electrodes to be selectively energized by a ribbon electron beamimpinging thereon, electrostatic deflection plates intermediate saidelectron gun and said code plate for deflecting a ribbon electron beamwith respect to said code plate, dynamic tilt correction rodsintermediate said electron gun and said deflection plates formaintaining alignment between said code plate and an electron beamdeflected by said deflection plates, said rods being adapted to producea varying electric field across the width of a ribbon beam passingtherebetween, a voltage attenuating network connecting said rods andsaid deflection plates for applying to said rods a voltage dependent onthe voltages applied to said deflection plates; and an electrostaticdiverging lens intermediate said deflection plates and said code plate,said lens comprising a planar grid having a plurality of closely spacedelongated apertures, the long dimension of said apertures being alignedin the direction in which beam divergence is to be produced, a planarconductive member spaced from and parallel to said grid, said memberhaving a ribbon beam passing aperture therethrough, and means formaintaining a potential difference between said grid and said member.

References Cited in the file of this patent UNITED STATES PATENTS2,439,504 Broadway Apr. 13, 1948 2,524,606 Shelton Oct. 3, 19502,538,669 Coetrier Jan. 16, 1951 2,616,060 Goodall Oct. 28, 19522,642,547 Rodenhuis June 16, 1953 2,734,147 Beckers Feb. 7, 19562,758,235 Evans Aug. 7, 1956 2,855,540 Hoover et al. Oct. 7, 19582,916,660 Ketchledge Dec. 8, 1959

1. AN ELECTRON BEAM TUBE COMPRISING TARGET MEANS, AN ELECTRON GUN FORPROJECTING A RIBBON ELECTRON BEAM IN AN INITIAL PLANE TOWARD SAID TARGETMEANS, ELECTROSTATIC DEFLECTION MEANS INTERMEDIATE SAID GUN AND SAIDTARGET MEANS FOR DEFLECTING SAID BEAM AWAY FROM SAID PLANE IN ADIRECTION PERPENDICULAR TO SAID PLANE, ELECTROSTATIC DIVERGING MEANSINTERMEDIATE SAID DEFLECTION MEANS AND SAID TARGET MEANS FOR CAUSINGSAID BEAM TO DIVERGE IN THE DIRECTION OF DEFLECTION, DYNAMIC TILTCORRECTION MEANS INTERMEDIATE SAID GUN AND SAID DEFLECTION MEANS FORMAINTAINING THE ALIGNMENT OF SAID BEAM AND SAID TARGET AS SAID BEAM ISDEFLECTED, SAID TILT CORRECTION MEANS COMPRISING CONDUCTIVE MEMBERSPOSITIONED ON OPPOSITE SIDES OF THE PLANE OF SAID RIBBON ELECTRON BEAM,AND MEANS FOR SUPPLYING TO SAID DYNAMIC TILT CORRETION MEANS POTENTIALSOF A MAGNITUDE FUNCTIONALLY DEPENDENT ON THE AMOUNT BY WHICH SAID BEAMIS DEFLECTED.